1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a backlight unit and a liquid crystal display device having the same that include a lamp with a bent shape to prevent a decrease in luminance.
2. Discussion of the Related Art
A cathode ray tube (CRT) device has been commonly used in televisions, measurement instruments, and information terminals, but a CRT device has a relatively heavy weight and a large size. Thus, a CRT device cannot satisfy the current demands of small-size and lightweight.
Hence, flat display devices, such as a liquid crystal display (LCD) device using an electric field optical effect, a plasma display panel (PDP) device using a gas discharge and an electroluminescence display (ELD) device using an electric field luminous effect, have been actively studied to substitute CRT devices. Among such display devices, LCD devices have been most actively studied because LCD devices have advantageous characteristics such as compact size, lightness and low power consumption. LCD devices have wide applications as ultra-thin flat display devices, for example, monitors for spacecrafts, aircrafts, notebook computers, laptop computers, desktop computers and large-sized display devices. Accordingly, the demand for LCD devices is continuously increasing.
In general, LCD devices control light transmittance from ambient light to display images. However, because an LCD device is non-self-luminous, it needs an external light source, such as a backlight unit. Such a backlight unit includes a cylindrical fluorescent lamp and is classified in one of two types, a direct type and an edge type, according to the arrangement of the fluorescent lamp.
In an edge type backlight unit, a lamp unit is provided at one side of a light-guiding plate. The lamp unit includes a lamp for emitting light, a lamp holder being inserted into both ends of the lamp to protect the lamp, and a reflective sheet having one side inserted to the light-guiding plate and surrounding the circumference of the lamp for reflecting light emitted from the lamp to the light-guiding plate. Thus, the edge type backlight unit is generally applied to relatively small sized LCD devices, e.g., monitors of a laptop computer and a desktop computer, because it is advantageous in that it has great uniformity of light, long life span, and thin profile of the LCD device.
With the trend of large-sized LCD devices of 20-inch or more, a direct type backlight unit is actively developed. In the direct type backlight unit, a plurality of lamps are formed along one line on a lower surface of a light-diffusion sheet, whereby the entire surface of the LCD panel is directly illuminated by light from the lamps. The direct type backlight unit, which has greater light efficiency as compared with an edge type backlight unit, is used for large-sized LCD devices requiring high luminance, e.g., large-sized monitors or televisions. A large-sized monitor or a television may be driven for a long time. Thus, one of the lamps of a direct type backlight unit may be broken while other lamps remain on, thereby causing a difference in luminance.
Accordingly, if one of the lamps is turned off due to malfunctions or the end of the lamp life, a screen portion corresponding to the turned-off lamp portion becomes darker as compared with surrounding portions of the screen. In this respect, it is necessary for the direct type LCD device to have a simple structure suitable for disassemble and assemble of the lamp unit.
FIG. 1 is an exploded perspective view illustrating an edge type backlight unit of an LCD device according to the related art. In FIG. 1, an edge type backlight unit includes an LCD panel 14, a lamp 10, a light-guiding plate 11, a light-diffusion sheet 12, a prism sheet 13, a fixation structure 15, and a lower reflective sheet 16. The LCD panel 14 displays images by using the lamp 10 as a light source and by controlling light transmittance thereof. Also, the light-guiding plate 11 guides light emitted from the lamp 10 to the LCD panel 14. Then, the light-diffusion sheet 12 diffuses the light guided by the light-guiding plate 11 to a predetermined angle. The prism sheet 13 concentrates the diffused light and sends the concentrated light to the rear of the LCD panel 14. The fixation structure 15 is positioned below the light-guiding plate 11. To minimize light loss, the lower reflective sheet 16 reflects the light progressing toward the fixation structure 15 to the LCD panel 14.
In addition, the edge type backlight unit according to the related art further includes lamp reflective sheets 18 and lamp holders 17. Each of the lamp reflective sheets 18 covers the lamp 10 except for a light-incidence surface of the light-guiding plate 11, thereby decreasing the loss of light incident on the light-incidence surface of the light-guiding plate 11 from the lamp 10. Also, the lamp holders 17 are positioned at both ends of the lamp 10 to secure the lamp 10 at a predetermined position and to maintain a predetermined interval between the light-incidence surface of the light-guiding plate 11 and the lamp 10. Further, the plurality of light-diffusion sheets 12 and prism sheets 13 may be deposited as needed.
If the edge type backlight unit according to the related art is used for the monitor, each one lamp 10 is positioned at both sides of the light-guiding plate 11. However, in case the edge type backlight unit is used for the notebook computer, the lamp may be positioned at any one side of the light-guiding plate 11.
FIG. 2 is an exploded perspective view illustrating a direct type backlight unit of an LCD device according to the related art. Referring to FIG. 2, a direct type backlight unit according to the related art includes a plurality of fluorescent lamps 1, an outer case 3, and light-scattering means 5a, 5b and 5c. The plurality of fluorescent lamps 1 are arranged at fixed intervals along one direction, and the outer case 3 fixes and supports the plurality of fluorescent lamps 1. The light-scattering means 5a, 5b and 5c are provided between the fluorescent lamps 1 and an LCD panel (not shown).
The light-scattering means 5a, 5b and 5c prevent the silhouette of the fluorescent lamps 1 from being reflected on the display surface of the LCD panel (not shown), and provide a light source with uniform luminance. For improving the light-scattering effect, there are a plurality of diffusion sheets and one diffusion plate between the fluorescent lamps 1 and the LCD panel. Also, a reflective sheet 7 is provided on an inner surface of the outer case 3 for concentrating the light emitted from the fluorescent lamps 1 to the display part of the LCD panel, thereby improving light efficiency.
The fluorescent lamps 1 are formed of cold cathode fluorescent lamps (CCFLs) having electrodes formed at both ends of a tube. Accordingly, when a voltage is applied to the electrodes of the fluorescent lamps 1, the fluorescent lamps 1 emit the light. Also, the both ends of the fluorescent lamp 1 are respectively inserted to holes formed on both sides of the outer case 3.
The both electrodes of the fluorescent lamp 1 are connected with power supplying wires 9 and 9a for transmitting the power of operating the fluorescent lamp 1. Also, the power supplying wires 9 and 9a are connected to a driving circuit by an additional connector. Thus, each fluorescent lamp 1 requires an individual connector. For example, the power supplying wire 9 connected to one electrode of the fluorescent lamp 1 and the power supplying wire 9a connected to the another electrode of the fluorescent lamp 1 are connected to one connector. Also, any one of the power supplying wires 9 and 9a is curved to the lower side of the outer case 3 for being connected with the connector.
FIG. 3 is a plane view illustrating another direct type backlight unit of an LCD device according to the related art. As shown in FIG. 3, a direct type backlight unit uses a plurality of U-shaped fluorescent lamps 31. In each of the U-shaped fluorescent lamps 31, electrodes are formed at the both ends and connected to respective connectors 33a and 33b of an inverter 32. The backlight unit also includes an outer case 30 for securing the fluorescent lamps 31.
However, the edge type backlight unit and the direct type backlight unit according to the related art have the following disadvantages. First, although the edge type backlight unit according to the related art can improve the uniformity of light with the light-scattering means 5a, 5b and 5c, the edge type backlight unit is not capable of partially controlling display luminance because it uses a light-guiding plate.
Similar to the edge type backlight unit, the direct type backlight unit according to the related art also has a limitation in partial control of display luminance. Further, if the U-shaped fluorescent lamps are partially overlapped one another, the luminance is partially lowered in the direct type backlight unit according to the related art.